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Energy & Fuels 2006, 20, 2616-2622
Integrated Catalytic Process for Conversion of Biomass to Hydrogen Sadashiv M. Swami and Martin A. Abraham* Department of Chemical and EnVironmental Engineering, The UniVersity of Toledo, Toledo, Ohio 43606 ReceiVed February 9, 2006. ReVised Manuscript ReceiVed August 30, 2006
Catalytic steam and autothermal reforming of glucose, glycerol, and industrial wastewater was evaluated as a basis for the development of a process for the production of hydrogen from renewable feedstocks. A catalyst containing copper, nickel, and palladium was found to be effective under atmospheric pressure within the temperature range of 500-800 °C. The effects of the steam-to-carbon and air-to-fuel ratios on the char formation rates were studied. The problem of char formation was found to be predominant for the relatively high concentrations required for a commercially viable process and used for the feed streams of the pure components. However, two different industrial wastewaters obtained from the potato industry and the brewing industry were partially converted to hydrogen through steam reforming using this Ni/Pd/Cu catalyst, without noticeable char formation. The results obtained during reforming of the wastewater were compared with the results obtained during autothermal reforming of the pure species.
Introduction Conversion of biomass to hydrogen is potentially an economically viable opportunity to generate hydrogen while utilizing domestically available resources. The steam reforming of biomass and biomass-derived materials has been considered as an alternative source of renewable energy. Steam reforming of ethanol for hydrogen production has been shown to be entirely feasible from a thermodynamic point of view.1-3 The reforming of simple biomass-derived compounds such as ethanol, methanol, and methane has been already studied. Previous research4,5 has shown that a copper-nickel catalyst supported on γ-Al2O3 and doped with potassium hydroxide is suitable for the production of synthesis gas from steam reforming of alcohols. Copper based catalysts have been used at 200300 °C for methanol,6,7 while nickel8 at 600 °C, rhodium9 supported on Al2O3 at 650 °C, and copper/nickel blends10 at 300 °C have been used for steam reforming of ethanol. Previously, steam reforming also has been demonstrated for hydrogen production from biomass and cellulose11,12 using a reduced nickel catalyst at 200-350 °C to produce a product gas containing CH4, H2, and CO2. * Corresponding author. Phone: (419) 530-4986. Fax: (419) 530-4727. E-mail:
[email protected]. (1) Vasudeva, K.; Mitra, N.; Umasankar, P.; Dhingra, S. C. Int. J. Hydrogen Energy 1996, 21, 13. (2) Maggio, G.; Freni S.; Cavallaro, S. J. Power Sources 1998, 74, 17. (3) Fishtik, I.; Alenxander, A.; Datta R.; Geana, D. Int. J. Hydrogen Energy 2000, 25, 31. (4) Luengo, C.; Ciampi, G.; Steckelberg, C.; Laborde, M. Int. J. Hydrogen Energy 1992, 17 (9), 677. (5) Marino, F. J.; Cerrella, E. G.; Duhalde, S.; Jobbagy, M.; Laborde, M. A. Hydrogen from steam reforming of ethanol. Characterization and performance of copper-nickel supported catalysts. Int. J. Hydrogen Energy 1998, 23 (12), 1095-1101. (6) Lindstro¨m, B.; Pettersson, L. J. Int. J. Hydrogen Energy 2001, 26, 923. (7) Agrell, J.; Birgersson, H.; Boutonnet, M. J. Power Sources 2002, 106, 249. (8) Fatsikostas, A. N.; Kondarides, D. I.; Verykios, X. E. Catal. Today 2002, 75 (1-4), 145. (9) Cavallaro, S.; Chiodo, V.; Vita, A.; Freni, S. J. Power Sources 2003, 123 (1), 10. (10) Marin˜o, F. J.; Cerrella, E. G.; Duhalde, S.; Jobbagy, M.; Laborde, M. A. Int. J. Hydrogen Energy 1998, 23, 1095.
The most commonly used techniques for biomass reforming are gasification and pyrolysis. Biomass pyrolysis, in which biomass is heated and decomposed in an inert atmosphere or under a vacuum, has been reviewed by Antal.13 Pyrolysis of cellulosic carbohydrates leads to variety of smaller compounds, including liquid organics, light gases, and char.13,14 Pyrolysis of glucose and subsequent steam reforming of fast-pyrolysis oil has been investigated for the production of hydrogen gas at 550-810 °C using Ni based catalyst.15 The disadvantage of pyrolysis is the decomposition of the feedstock, resulting in the formation of char. Pyrolysis of biomass produces bio-oil and glycerol,16 which can subsequently be used in steam reforming to obtain hydrogen. Steam gasification of cellulose, lignin, and bagasse has been reported at 700 °C, and the effect of the heating rates on the formation of the char has been studied. It was reported that there is no pronounced influence of heating rate on the elemental composition of char, but higher rates substantially increased the reaction rate of char.17 Gasification of glucose in supercritical water at 600 °C, 34.5 MPa, and 30 s residence time results in formation of the H2, CO2, CO, and CH4.18 Alternatively, hydrogen can be directly obtained from aqueous-phase reforming of glucose without formation of char.19 The aqueous-phase reforming reactions of glucose were only successful at very low concentrations of glucose (1%), whereas high biomass concentration will be necessary to reach commercial goals. However, the results of experiments until now indicate that high concentrations (11) Courson, C.; Makaga, E.; Petit, C.; Kiennemann, A. Catal. Today 2000, 63, 427. (12) Demirbas, A. Energy ConV. Manage. 2002, 43, 897. (13) Antal, M. J., Jr. AdV. Solar Energy 1982, 1, 61. (14) Evans, R. J.; Milne, T. A. Energy Fuels 1987, 1 (2), 123. (15) Marquevich, M.; Czernik, S.; Chornet, E.; Montane, D. Energy Fuels 1999, 13, 1160. (16) Davada, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal., B 2003, 43, 13. (17) Fushimi, C.; Araki, K.; Yamaguchi, Y.; Tsutsumi, A. Ind. Eng. Chem. Res. 2003, 42 (17), 3922. (18) Yu, D.; Aihara, M.; Antal, M. J., Jr. Energy Fuels 1993, 7 (5), 574. (19) Davada, R. R.; Dumesic, J. A. Chem. Commun. (Cambridge, U.K.) 2004, 1, 36.
10.1021/ef060054f CCC: $33.50 © 2006 American Chemical Society Published on Web 10/14/2006
Catalytic Process for ConVersion of Biomass to Hydrogen
Figure 1. Process concept for conversion of biomass resources to hydrogen showing integrated biological and catalytic process steps.
of biomass cause a low gasification efficiency plus liquid and solid carbon effluents.16 Because of the limitations found in the reforming of solid biomass feedstocks, we envision an integrated scheme, similar to that shown in Figure 1, in which conventional conversion processes based on anaerobic digestion and catalytic steam reforming are combined to convert waste biomass into hydrogen. Biomass in food processing and agricultural waste can be converted, nearly eliminating the cost of raw materials (or even possibly generating revenue by reducing waste-disposal related costs) and improving the overall economics of the process. In order to be a practical option for agricultural interests, an economically viable process must be developed for converting target waste streams to hydrogen. The advantage of the proposed process is that it capitalizes on the strengths of each processing unit and minimizes the need for these existing technologies to be substantially modified for highly unconventional feed or product requirements. The current interest in energy security and the use of biomass resources provides motivation to study steam and autothermal reforming of biomass at higher concentrations. Since the glucose molecule contains the same functional group as the cellulose polymer, it is an appropriate model compound for biomass feedstocks that will mimic the reaction chemistry of many carbohydrates that (together with lignin) compose biomass. The evaluation of glucose and biomass conversion using a Ni/Pd/ Cu/K based catalyst is presented.
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was controlled using the mass flow controller (Alicat Scientific, Inc., M-1SLPM-D). All feed streams were passed through the preheater, the temperature of which was controlled below the reactor temperature and above the boiling point of the liquid solution. The streams were then mixed together and allowed to pass to the reactor. The product gases were passed through the glass condenser, in which the condensable products were separated from the noncondensable products. Noncondensed gases were allowed to go through a Shimadzu 2010 gas chromatograph, equipped with both a PDID (pulse discharge ionization detector) and an FID (flame ionization detector), for real-time analysis. The flow rate of the product gases was measured using the Alicat Scientific flow meter (model # V-2LPM-D) mounted on the product gas line between the condenser and the GC. The flow meter was attached to a computer through an RS-232 port, and a flow rate reading was recorded once per second. The volumetric flow rate was measured in H2 mode and then converted to actual volumetric flow rates based on the estimated viscosity of the gas mixture and H2 gas. This correction included calibration for 14 different gases in order to adjust for the composition of the product stream. The digital flow meter also measured temperature and pressure so that the measured volumetric flow rates could be converted to standard conditions. Char that was produced during the reaction was collected at the end of the experiment and weighed. The catalysts used in this study were prepared by the sequential wet impregnation technique. The catalyst metals were supported on 0.6-1.0 mm γ-Al2O3 spherical pellets (Almatis Inc). Potassium was impregnated from a 1.036 N KOH solution by placing the particles into the liquid solution and evaporating the liquid overnight in an oven at 80 °C, followed by calcination in an inert atmosphere at 800 °C for 2 h. The other metals (Pd, Cu, and Ni) were then impregnated onto this potassium-treated support from a solution containing 247.3, 35.74, and 58.72 mg/ mL of nickel(II) nitrate hexahydrate, palladium(II) nitrate hydrate, and copper(II) nitrate trihydrate, respectively (all obtained from Sigma-Aldrich and used as received). The catalyst was then calcined in air at 800 °C for 2 h to produce the final catalytic species and used without reduction for study of reforming reactions. The final nickel, palladium, copper, and potassium loadings were estimated from the amounts of metals included in the solutions from which they were impregnated and the weights of Al2O3 support used.
Materials and Methods Reaction experiments were performed in the reactor systems available in our laboratory, as shown in Figure 2 parts a and b. For all experiments other than those for glucose, the reactants were fed from the bottom of the reactor as shown in Figure 2a. For experiments with glucose, the feed was added from the top as shown in Figure 2b, in an attempt to avoid clogging of the tubing in the preheater. An ISO 1000 isocratic pump (Chrom Tech, Inc.) was used to feed the liquid solution. All the experiments were carried out in a 1.75 cm i.d. and 45 cm long stainless steel tubular reactor. The reactor tube was placed in a furnace (Applied Test Systems, Inc.). The catalyst was placed in the middle of the tube and was supported by quartz wool at both ends. The reaction temperature was monitored using a K-type thermocouple inserted into the reactor tube and located immediately downstream of the catalyst bed. The temperature was controlled using an Omega temperature controller (SSRDIN660DC25 and CNi16D44-C4EI). For the autothermal reforming reactions, industrial-grade air with 99.5% purity (Airgas, Inc.) was used. The flow rate of air
Results and Discussion Steam and autothermal reforming of several bioderived compounds have been evaluated as indicators for the performance of biomass-derived aqueous feeds. The biomass-derived molecules used in this study can undergo reforming to CO, CO2, and H2 according to several simultaneous pathways, including:
Steam Reforming: CxHyOx + xH2O f xCO2 + (x + y/2)H2
(1)
Decomposition in the presence of water: CxHyOx f xCO + y/2H2
(2)
Water gas shift reaction: CO + H2O T CO2 + H2
(3)
Finally, in the presence of oxygen, combustion reactions may
2618 Energy & Fuels, Vol. 20, No. 6, 2006
Swami and Abraham
Figure 2. (a) Experimental setup used for reforming of glycerol and wastewater, showing upward flow through reactor. (b) Experimental setup used for autothermal reforming of glucose, showing downward flow of all reactants (symbols as defined in Figure 2a).
also occur:
CO + 1/2O2 f CO2
(4a)
H2 + 1/2O2 f H2O
(4b)
The case of autothermal reforming can be described as a combination of steam reforming (reaction 1) plus CO combustion (reaction 4a), with the specific product composition and the amount of CO produced dependent on the relative rates of the two reactions. We note that other combinations of reaction pathways could be constructed to describe all possible products;
however, reactions 1-4 provide an appropriate set of reaction pathways that can be combined to describe all possible reaction routes. In order to evaluate the performance of the catalyst, the yields of H2 and CO2 were calculated as actual moles of H2 (or CO2) produced relative to the stoichiometric maximum number of moles of H2 (or CO2) possible according to the steam reforming reaction 1.
% yield )
(Fi) (νi)(Fj,0)
× 100
(5)
Catalytic Process for ConVersion of Biomass to Hydrogen
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Figure 3. Product gas composition (dry basis) obtained during steam reforming of glycerol (S/C ) 3) over a Pd/Ni/Cu/K catalyst as a function of reaction temperature.
where Fi is the moles of species i produced per min, νi is the stoichiometric coefficient of species i, and Fj,0 is the molar flow rate of the reactant in mol/min. This measure of yield describes both the conversion of the reactant and the selectivity for the reforming reaction and is, thus, believed to be an appropriate measure of the catalyst effectiveness. Glycerol Reforming Because of the known difficulties associated with char formation during reforming of glucose, glycerol was first evaluated as an alternate surrogate material. Glycerol (C3H8O3) is an important biomass-derived product, as it is a major byproduct of the bio-oil production process. Steam reforming was accomplished at an S/C ratio of 3, whereas autothermal reforming was done at a steam-to-carbon ratio (S/C) of 3 and an oxygento-carbon ratio (O/C) of 0.3. The effect of temperature on the hydrogen production during steam and autothermal reforming has been evaluated in the range of 550-850 °C using the Pd/ Ni/Cu/K catalyst prepared in our laboratory. The reactor was operated under isothermal conditions. Mear’s criterion was employed to confirm the isothermal conditions in the reactor.20 Mear’s criterion proposed that the bulk fluid temperature, T, will be virtually the same as the temperature at the external surface of the pellet when
|
|
-∆Hrx(-r′A)FbREA hT2Rg
< 0.15
(6)
In fact, the value of the modulus was calculated to be 0.002 at 550 °C, confirming that there was no temperature gradient in catalyst pellets. McAdam’s correlation was used for the estimation of the heat transfer coefficient.21 Figure 3 provides the percentage composition (mole percent on a dry basis) of the gas products as a function of reaction temperature during steam reforming of glycerol. According to reaction 1, complete steam reforming of glycerol could provide a gas with a maximum hydrogen content of 70%, while reaction 2 suggests glycerol decomposition could lead to a hydrogen content of 57%. Our results indicate that the gas contained between 50 and 60% hydrogen, with slightly higher hydrogen concentrations at higher temperatures. CO is the dominant carbon product, particularly at lower temperatures. At temperatures of 700 °C and above, CO2 and CO compositions are (20) Fogler, H. S. Elements of Chemical Reaction Engineering, 3rd ed.; Prentice Hall PTR: Upper Saddle River, NJ, 2001; p 761. (21) Holman, J. P. Heat Transfer, 8th ed.; McGraw-Hill: New York, 1997; p 307.
Figure 4. Comparison of H2 yield obtained during steam reforming (S/C ) 3) and autothermal reforming (S/C ) 3, O/C ) 0.3) of glycerol over a Pd/Ni/Cu/K catalyst, as a function of reaction temperature.
roughly equivalent. Some methane is also produced in low concentration (
